The components of high-temperature mechanical systems, such as, for example, gas-turbine engines, must operate in severe environments. For example, the high-pressure turbine blades and vanes exposed to hot gases in commercial aeronautical engines typically experience metal surface temperatures of about 1000° C., with short-term peaks as high as 1100° C. A portion of a typical metallic article 10 used in a high-temperature mechanical system is shown in FIG. 1. The blade 10 includes a Ni or Co-based superalloy substrate 12 coated with a thermal barrier coating (TBC) 14. The thermal barrier coating 14 includes a thermally insulative ceramic topcoat 20 and an underlying metallic bond coat 16. The topcoat 20, usually applied either by air plasma spraying or electron beam physical vapor deposition, is most often a layer of yttria-stabilized zirconia (YSZ) with a thickness of about 300–600 μm. The properties of YSZ include low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion. The YSZ topcoat 20 is also made “strain tolerant” by depositing a structure that contains numerous pores and/or pathways. The consequently high oxygen permeability of the YSZ topcoat 20 imposes the constraint that the metallic bond coat 16 must be resistant to oxidation attack. The bond coat 16 is therefore sufficiently rich in Al to form a layer 18 of a protective thermally grown oxide (TGO) scale of α-Al2O3. In addition to imparting oxidation resistance, the TGO bonds the ceramic topcoat 20 to the substrate 12 and bond coat 16. Notwithstanding the thermal protection provided by the thermal barrier coating 14, the spallation and cracking of the thickening TGO scale layer 18 is the ultimate failure mechanism of commercial TBCs. Thus, improving the adhesion and integrity of the interfacial TGO scale 18 is critical to the development of more reliable TBCs. Related to this is the need to significantly reduce the progressive roughening or “rumpling” of the bond coat surface during thermal exposure, which is a formidable limitation of conventional bond coat systems.
The adhesion and mechanical integrity of the TGO scale layer 18 is very dependent on the composition and structure of the bond coat 16. Ideally, when exposed to high temperatures, the bond coat 16 should oxidize to form a slow-growing, non-porous TGO scale that adheres well to the superalloy substrate 12. Conventional bond coats 16 are typically either an MCrAlY overlay (where M=Ni, Co, NiCo, or Fe) or a platinum-modified diffusion aluminide (β-NiAl-Pt). The Al content in these coatings is sufficiently high that the Al2O3 scale layer 18 can “re-heal ” following repeated spalling during service of the turbine component.
However, the adhesion, and therefore the reliability, of the TBC system is measured with respect to the first spallation event of the TGO scale layer 18. As a result, once the first spallation event occurs in the scale layer 18, the ceramic topcoat 20 can begin to delaminate and fail, so that re-healing of the scale layer 18 is not a critically important performance requirement for the adhesion of the ceramic topcoat 20. Thus, conventional bond coats, which were designed primarily for re-healing the Al2O3 TGO scale layer, do not necessarily possess the optimum compositions and/or phase constitutions to provide enhanced scale layer adhesion and improved TBC reliability.
Another approach to improving the adhesion of the TGO scale layer on a second metallic article 28 is shown in FIG. 2A. A superalloy substrate 30 is coated on an outer surface with a layer 32 of Pt and then heat-treated. Referring to FIG. 2B, following this heat treatment Al diffuses from the superalloy substrate 30 into the Pt layer 32 to form a surface-modified outer region 34 on the superalloy substrate (FIG. 2B). An Al2O3 TGO scale layer 38 and a ceramic layer topcoat 40 may then be formed on the surface modified region 34 using conventional techniques. However, since transition metals from the superalloy substrate 30 are also present in the surface modified region 34, it is difficult to precisely control the composition and phase constitution of the surface region 34 to provide optimum properties to improve adhesion of the TGO scale layer 38.
Future improvements in gas-turbine performance will require even higher operating efficiencies, longer operating lifetimes, reduced emissions and, therefore, higher turbine operating temperatures. Improved TBCs are needed to protect turbine operating components at increased temperatures (e.g. 1150° C.), and new bond coat compositions must be developed to reduce spallation and increase adhesion of the TGO layer, which will result in an enhanced reliability for the ceramic topcoat layer.